1. Field of the Invention
The present invention relates to an X-ray image intensifier for converting an X-ray image to an optical image.
2. Description of the Related Art
An X-ray image intensifier is commonly used for medical or industrial purposes, and widely used for diagnosis or non-destructive inspection. A conventional X-ray image intensifier, such as that shown generally in FIG. 1, has an X-ray input window 11 and a vacuum vessel 12 having a glass barrel. An X-ray input screen 13, for emitting electrons in accordance with an input X-ray, is formed on the X-ray input window 11. The input screen 13, which in general has a fluorescent material and a photocathode, serves as a cathode of the X-ray image intensifier. On the opposite side of the input screen in the vacuum vessel 12 is formed an output screen 14, having a fluorescent material layer which emits light upon collision of electrons. A plurality of focusing electrodes 15 and 16 are arranged at predetermined portions in the vacuum vessel for forming an electrostatic electron lens and an accelerating anode 18 is arranged near the output screen.
When the X-ray image intensifier is operated, an operating power source (not shown) supplies potentials to the above elements: for example, a ground potential to the input screen 13, +300 V to a first focusing electrode 15, +1.7 kV to a second focusing electrode, and +30 kV to the output screen 14 and the anode 18. As a result, an X-ray emitted from an X-ray generator (not shown) is input to the input screen through an object and an X-ray image of the object is converted to a fluorescent image, which is further converted to an electronic image by the photocathode. The electronic image is accelerated and focused by an electronic lens system constituted by the focusing electrodes and the anode. The focused image, causing the fluorescent material layer to emit, is converted to a visible image having an increased conversion factor and output from the image intensifier.
When the X-ray image intensifier is used in a state where a very small amount of X-ray is input, a relatively small number of electrons are generated from the input screen and the amount of gas generated in the vacuum vessel is also small. It is therefore possible to maintain the vessel in a high vacuum for a long period of time merely by incorporating a predetermined amount of getters in the vessel, so that a stable operation can be obtained.
However, when the X-ray image intensifier is exposed to a relatively large amount of X-ray, as in a non-destructive inspection, a large amount of electrons are generated from the input screen and a large amount of gas is generated from the electrodes in the vessel, the output screen, the other elements in the vessel, and the vacuum vessel itself. As a result, the electron emitting performance of the input screen may be degraded or an abnormal discharge may occur in the vessel.